A problem has been encountered in the construction of signs having three-dimensional characters. The face of the character generally is translucent plastic and the sides of the character are a sheet metal strip, and effectively joining the plastic and sheet metal strip has proved difficult. Plastic can not be fixed directly to the sheet metal strip, but requires an adhesive or mechanical joint.
Regarding adhesive joints between the circumscribing sheet metal strip and the typically translucent plastic character face, this solution has not been satisfactory. It has been found quite difficult to simply glue the thin plastic face directly to the sheet metal strip and achieve a bond with acceptable strength and adhesion. In most three-dimensional signs only the edge of the translucent plastic face is in contact with the metal shell. The translucent face is generally quite thin, even for a relatively large sign, rarely exceeding 1/4 inch. This presents only a relatively small surface-to-surface connection between the translucent plastic face and the sheet metal strip shell, which greatly decreases the possible strength of an adhesive bond therebetween. In gluing the face directly to the sheet metal strip shell, the situation is aggravated by the presence of the face against the metal which prevents the penetration of glue across the face where it abuts the sheet metal strip siding. This prevents the formation of an adhesive bond across the entire thickness of the abutted face. These difficulties have heretofore prevented the sole use of an adhesive bond to attach the plastic face to the metal shell, since a bond of adequate strength simply cannot be formed in this manner with glues presently available.
A possible solution to the above problems has been to apply adhesive two times to the metal sheet strip. In the first application, the adhesive was spread and allowed to form a cured layer prior to attachment of the plastic face. In the second application, adhesive on the abutted edge of the plastic sign face was bonded on the previously cured layer of adhesive.
This solution has not been satisfactory. When adhesive is applied to attach the plastic face, the adhesive and its solvent attach the cured layer and destroy the continuity thereof and result in a bond surface which is insufficient to provide the desired strength.
Regarding mechanical joints, the plastic face is usually so thin that it is quite difficult to directly attach the plastic to the sheet metal strip with screws or bolts. L-shaped brackets could be used, but are unacceptable since they interfere locally with the visual qualities of the sign. Typically, such brackets locally obscure or block light passing through or reflected from the translucent sign face resulting in an irregularly appearing or illuminated and hence unacceptable sign product. Various arrangements of slots and grooves have been attempted with limited success.
The complexity of the contrivances which have been necessary in the past to form an effective joint between the plastic face and the sheet metal are perhaps best illustrated by the patent to Minogue, U.S. Pat. No. 3,414,305. In that patent, the sheet metal was rolled into convolutes, and an intermediate plastic piece was compressed within the convolutes, typically in the form of a "Pittsburgh lock". The plastic face was then bonded to the intermediate plastic piece. While this yielded a mechanical bond between the plastic face and the sheet metal, a relatively complex and expensive process was required to form the convolutes in the metal and compress the intermediate plastic piece therein. Moreover, the ultimate sign product left much to be desired.
Mechanical locks between sheet metal rolled into convolutes and intermediate plastic stripping have several disadvantages. First, such locks can only be formed on metal strips of finite length--typically in the order of 10 to 12 feet--on a machine known as a metal brake. Numerous repetitive sequential operations of the metal brake are required, making the cost of the mechanical bond between the metal and plastic strip expensive.
Secondly, the plastic strip pulls away from its backing, even when ordinary gluing of the strip to the metal strip is attempted. This occurs after the translucent face of the sign has been secured to the circumscribing sheet metal strip sides of the sign. Typically, dirt lodges in between the metal strip and the plastic strip held to the metal by the mechanical lock in the manner illustrated in FIG. 3. This dirt forms an unsightly border to the sign display, rendering either its expensive cleaning or replacement necessary.
Additionally, the mechanical lock at its edge includes multiple overlying layers of material. In the case of the Pittsburgh lock, three overlying layers of material are composed of the metal of the sign strip, and two overlying layers of material are formed of plastic, for a total of five overlying layers. This combined five overlying layer thickness of metal and plastic cannot be conveniently bent, especially to right or acute angles without cracking or tearing of at least one of the metal layers or at least one of the plastic layers, or both. This often weakens the construction of the sign at its most critical areas.
Finally, the Pittsburgh lock is not acceptable for production on a rolling mill. Typically, attempts to compress multiple layers of material on a rolling mill with sufficient compression to form a mechanical lock results in elongation of the edges of the circumscribing sheet metal strip with respect to the central portion of the circumscribing sheet metal strip. This produces a sinusoidal curve of the edges relative to the central portion of the metal strip. This sinusoidal curve of the edges of the metal strip renders impossible the attachment of the edge of the circumscribing sheet metal strip to the translucent sign face.
Moreover, when the product is formed on a metal brake as distinguished from a rolling mill, uneven width of the circumscribing metal strip results. As opposed portions of signs require precisely correspondent thicknesses of circumscribing metal strip, an unsatisfactory sign product results.